The present invention relates to airborne Internet and specifically to a low-latency/low link margin airborne satellite Internet system using commercial off-the-shelf infrastructure (COTS).
The present application is related to U.S. Pat. No. 6,529,706 “Aircraft satellite communications system for distributing Internet service from direct broadcast satellites” by James P. Mitchell, assigned to the assignee of the present application, and is incorporated by reference in its entirety.
Systems for delivering Internet service from direct broadcast satellites to ground-based users are commercially available such as Hughes DIRECPC and DIRECWAY and from STARBAND Communications Inc. A future satellite service, Hughes SPACEWAY, is planned for delivery of Internet services. A direct broadcast satellite Internet system 100 is shown in FIG. 1. In a DIRECPC or similar system a client PC (personal computer) 110 requests a URL using a IP (Internet protocol) protocol over a telephone system 120 with a modem. The URL request has a tunneling code, address, or similar identifier attached to it before it is sent to an Internet service provider (ISP) 125. The code tells the ISP 125 to forward the URL request over the Internet 130 to a network operations center (NOC) 140. The ISP 125 may be collocated with the network operations center 140. In usual Internet operations the URL request is sent to a requested Web site 135 instead of the network operations center 140. The network operations center 140 receives the URL request, removes the tunneling code, and then forwards the request over the Internet 130 to the appropriate Web site 135. The requested Web pages are then retrieved and sent to the network operations center 140. At the network operations center 140 the Web page data is uplinked to a direct broadcast satellite 150. From the DBS satellite 150 the data is then transmitted to a small vehicle-mounted dish 115 and DBS receiver 117 at the client PC 110. A split communications system such as this has the advantage of speeding up Internet data delivery over the rates available over conventional telephone lines. This is due to the low-band width URL request from the client PC 110 delivered over the conventional telephone system 120 at slow rates and high-bandwidth large Web pages being delivered over a high data rate satellite link.
Systems such as DIRECWAY and STARBAND are two-way systems that transmit a request from the PC 110 to the receiver 117, that also includes a transmitter, directly to the satellite 150. The satellite 150 relays the request to the NOC 140 that may also contain a hub ISP. The requested Web pages are then retrieved and set to the NOC 140 for transmission over the satellite 150 as before.
Global Internet coverage and access to an aircraft has been accomplished in some regions of the world using the two-way systems described but continues to face difficult technical problems in others. Variables such as antenna gain, satellite EIRP, receiver design, forward error correction choices, antenna pointing, antenna beamwidth, aircraft dynamics, etc. require very specialized designs and system approaches to make global Internet coverage work.
TCP/IP over two-way satellite communications links can build up very long latencies especially if the link is encountering high bit error rate problems. Aircraft dynamics also introduce moments of satellite blindness in marginal RF coverage regions such as extreme latitudes. Typically if non-repairable data errors occur, the protocol initiates additional automatic repeat requests (ARQs). If the errors continue, the request is repeated again and after several tries several undesired seconds of geostationary satellite round-trip delays have occurred. Geostationary satellite links have more than a 500-ms round trip time.
While custom hybrid protocol methods including specialized FEC (forward error correction, UDP (user datagram protocol) and others can be used to assist this problem, custom solutions are costly and likely involve interventions with NOC 140 design and operation, the aircraft communications infrastructure and the client application.
TCP protocol acknowledgement spoofing methods are sometimes used by industry to reduce the effective latency of a reliable satellite link however these methods require a cooperative NOC and custom hardware at both end of the link resulting in a non-COTS system. These methods do nothing significant to combat poor link margin conditions.
Custom designed system elements, data services, and satellite protocol spoofing methods are vastly more costly than similar ground COTS only systems and services. For example the popular COTS direct-to-home two-way Ku-band services such as DIRECWAY and STARBAND described above cost about $2 a day in comparison to connectivity services for airborne applications that can run orders of magnitude higher at just the broadband connectivity service level alone.
Therefore a need exists for better leverage of existing commercial satellite infrastructure for improved cost and improved services.